Solutions of tetraalkylsilicates



'---The first-method of 'a -mul-ti-molecular film amounting in general rto 'a'layer: approximately 40 molecules thick of an insoluble soapp This layer was subsequently "treatedwith acid whereby the metallic ion in each soap molecule was removed thereby leaving Patented Apr. 7, 1953 SOLUTIONS OF TETRAALKYLS ILICATES Harold R. Moulton, Southbridge, Mass., assignor to American Optical Company, Southbridge, Mass., a voluntary association of Massachusetts No Drawing. Application January 18, 1946, Serial No. 642,101, now Patent No. 2,482,684, dated September 20, 1949, which-is a division of application Serial No. 495,906, July 23, 1943, now Patent No. 2,474,061, dated June 21, 1949.

Divided and this applica rial No. 86,390

tion April 8, 1949, Se-

6 Claims. (01; 106-287) This invention relates to new and novel solutions of tetraalkylsilicates in organic solvents, said solutions being for use in methods of reducing the refiectionof light impinging upon the surface of an article whereby, in the case of a transparent article, a greater percentage of incident light will traverse said article and in the case of an opaque article will reduce the reflectivity of said articles, and in some instances simultane ously increasing the scratch resistance of the surfaces of said articles.

This application is a division of my co-pend- --ing application Serial No. 642,101, filed January 18, 1946, now Patent No. 2,482,684 and which -was a division of application Serial No. 495,906, "filed July 23, 1943, now matured into Patent No.

"2,474,061 bearing date of June 21, 1949, which 'Intheprior art, surface coatings have been produced for reducing reflections of light im- -=pinging .upon the surfaces by commonly known "methods.

consists in the application a skeleton structure whoseporosity wa under The reflection reduction obtained by fol- Another method which has, in spite of its "drawbacks, retained a considerable commercial application, is an outgrowth of vacuum evaporation technique In this method, a thin layer, .j. generally one quarter of a wave length thick, of =...-'a' .v'olatile salt. or compound of low index is evaporated by means ofheat in a vacuum'and a :deposit is produced upon the surface to be treat- ,ed., The compounds used are customarily lithium ;f luoride, calcium fluoride, cryolite, :or magnesium ;..-fluqride orlzsimllar compounds. These com-- usual precautions have to be taken to keepthis tendency within usable bounds.

pounds produce surface layers which, when very carefully controlled as to thickness, a control which is diflicult and which is subject to many large scale manufacture their use has been re stricted. In addition, these coatings sufier'from the serious drawback of being readily attacked by moist atmosphere and exposure to the at;-

- mosphere causes such coatings to deteriorate rapidly. It has been found necessary, in order; to increase the resistance to atmospheric attack and to the attack of moisture to bake these coatings at a relatively high temperature, a proceeding accomplished by risk of damage to the accurate optical parts and one which, in spite of the slight increase'in weathering resistance, still falls far short of the weathering resistance ofordinary glass surfaces. additional drawback that there is considerable tendency for them to appear hazy or cloudy These coatings suffer from the under certain conditions of illumination and un- Another prior art method of obtaining glass surfaces having reduced reflection consists inthe treatment of the surfaces with hydrofluoric, acid gas or with dilute aqueous solution of hydrofluoric acid. In this process, certain constituents of the glass are rendered soluble or volatile and are then subsequently abstracted from the surface leaving a film of lower index. Other selected 'solvents for certain of the compounds present in the glass may be used and have been used ;in

the prior art, such as alkalis, soluble phosphates,

' nitric acid and even harsh laundry soap. .It is glasses treated and also upon the character and I cleanliness of the glass surface itself. Under cernormally. be invisible are rendered visible to the disadvantage of the article treated. The usejof -the above described process; of course; is retain conditions also, surface defects which would stricted to glass. I

' It, therefore, is one of the primary objects of this invention to produce surface coatings which will-overcome the1above disadvantages and provide coatings having similar-optical character- :no further change.

and at the same time with extreme resistance to the attack of moist atmosphere and moisture itself, and in some instances more resistant to scratching and chemical attack.

In addition to the reduction of reflection of incident light the coatings as produced serve as chemical protection for the glass or plastic surfaces.

In following the teachings of the inventionthe article which is to be coated may be of a light transmitting nature such as glass plates, lenses,

prisms or other optical elements and similar elements formed of plastics or artificial resins. In this particular instance, the element, as stated above, is to have light transmitting properties.

In the present instance, the articles referredto are particularly of the type which are adapted for use in telescope systems, field glasses, binoculars, camera lenses, periscopes or for any other uses in which the light is adapted to impinge upon a surface of the article. The surfaces of said articles, in order to reduce surface reflection and to increase light transmission, are coated with a stable water insoluble coating of a material having a lower index of refraction than the substrate.

This is accomplished by-applying a layer of the solution, which uponevaporation, deposits such a layer on the glass or material substrate. The material which forms this layer need not be presfent in'the solution in the actual form in which itiflnally exists on the surface of the article but may be a decomposable compound which upon the removal of the solvent leaves a surface layer onthe substrate in the desired state, for example, a solution of from 1 to 10 per cent by volume of tetraethylorthosilicate, to 60 per cent by volume ethyl acetate, 1 to 10 per cent by volume concentrated hydrochloric acid'and the balance ethyl alcohol. Denatured alcohol may be substituted for a part or all of the ethyl alcohol. A preferred solution is five parts of tetraethylorthosilicate, 50 parts of ethyl acetate, 50 parts of denatured alcohol and 5 parts of concentrated hydrochloric acid, mixed in the order given. This gives a clear, colorless, stable solution. An essential'feature-ofthis solution is aging for several days before use. The exact reason for this is not known but it is believed to be a partial decomposition of the tetraethylorthosilicate by the hydrochloric acid resulting in an organo-sol of either hydrated or unhydrated silica. With the composition set forth above, an aging'of from two to six days is desirable. It has been found desirable for certain articles, such as lenses, especially those of steep curve, to spin the article at relatively slow speed and pour upon the upper surface thereof a small measured quantity of the solution. In this way, one surface may be done at a time and the smoothness and uniformity of the coating may be increased. In either case, the

spinning of the article is continued until the layer of solution formed thereby has evaporated sufficiently so that no drips or runs will occur when the article is removed from the holder. Prolonged aging to two or three months produces This aging normally takes place at room temperatures but it can be accelerated slightly by working at more elevated temperatures up to 50 C.

The solution having been prepared and aged, the procedure is as follows:

The lens or other article is immersed in this liquid, removed and. immediately spun in order "to remove excess liquid. Over a very wide range of translational velocities the effective thickness of the resulting layer is in the range for efllcient reflection reduction thereby differing from previous methods of obtaining reflection reduction in which the efliciency of the final product is greatly dependent upon the variation of thickness in the film. At this point, at least three alternative methods may be as follows:

In the first method the article is placed'in a. moist atmosphere at a temperature of approximately C. for a time interval suflicient to substantially heat the entire article to said tem-- perature after which the article may be cooled. This procedure renders the coating insoluble in acids or water and difiicultly soluble in alkaline solutions. The coating produced by this method reflects approximately 1% on glass of 1.52 index of light from each surface. In other words, for a flat piece of glass treated by this method substantially 98% of the incident light traverses it in contradistinction to the figure 91.8 which is the inherent transmission of glass'of this index of refraction. If the glass, for "exampleis of a higher index of refractionthan 1.52 the reduction of reflection is still greater. The treatment at 100 C. moist heat, serves to expedite the rendering insoluble of the surface reflection reducing coating. There is amore rapid increase in the hardness of this surface than occurs in the next two methods but the end result in all cases is substantially the same as regards hardness and insolubility. Although I have referred to the temperature as being 100C. it is to be understood that any suitable heating temperatures may be used and been found that a wider'range of temperatures may be used and produce surface coatings of substantially'similar characteristics, the top limit of such temperatures being set by the softening or distortion point of the material being treated, the lower temperature of 100 0. being preferable for use as it permits composite structures which are united by cementing or the like whiohare capable of withstanding this temperature but which would be susceptible to injury at higher temperatures. Lower temperatures, which are above room temperatures, are therefore preferably desirable. The temperature, therefore, must mainly be such as to produce coatings having desired characteristics without injury to the initial articles. The temperatures referred to hereinafter are therefore to be considered in the light of the above.

The second method consists in holding the article at room temperature after the solution has been applied and the excess removed by any convenient means such as the spinning asset forth above. The article is held at room temperature'in normally moist air for a period of approximately one minute, the time varying inversely with the ambient temperature. At the expiration of this exposure period the article is subjected to the action of water which may be cool or warm for any convenient period of time as it has been found that variations in the time of water treatment have substantially the same effect. This treatment somewhat increases the reflection reduction above mentioned but the article may be heated to 100 C, subsequently in order to insure permanency. It is felt that this gives a somewhat more porous surface and one which is somewhat less easy to clean.

The third method consists in simply allowing the article to stand at room temperature after the excess solution has been drained ofi or removed by spinning .in' which case the article I are desirable, with the article being so oriented asto drain in a different direction after each dip whereby the effectiveness of the coating is maintained substantially uniform over the article. This procedure is particularly useful in coating large sheets.

' Each coating may be heat treated, as above de: scribed, before the-application of the succeeding coating or the coating may be simply allowed to dry in air as described'in' method #3 or the final productmay be treated with water as described in method #2 or any'com'bination of these proceduresmay be followed either on each individual coating or on any one of the successive coatings.

In the'above example, tetraethylorthosilicate has been mentioned as has hydrochloric acid, denatured alcohol and ethyl acetate. It is, of course, understood that other alkyl silicates, such as tetramethylorthosilicate, or other esters of silicic acid may be used instead of tetraethylticular instance, for example, methyl acetate,

methyl alcohol, amylacetate, isopropyl alcohol, or in fact. most organic solvents may be used in which both the silicon ester and the acid are soluble. It is to be understood also that the organo-sol ofsilica may be separately prepared and purified and used in suitable solvents. I It has been found that other organic solvents than those enumerated maybe used to advantage; for example:

Ethylene glycol mono ethyl ether approximately 24%; ethyleneglycol mono butyl ether approximately 10%,"buty1 alcohol approximately 16%, denatured alcohol approximately 40%, tetraethylorthosilicate approximately 5%, and concentrated hydrochloric acid approximatet ,The relative proportions of these various in-, gredients may be varied in accordance with the method of application as lower speed of rotation during or subsequent to the application to the article to be coated will require a lower concentration of the active ingredient; namely the tetraethylorthosilicate. The high boiling solvents, such as ethylene glycol mono ethyl ether, ethylene glycol mono butyl ether and butyl alcohol may have their proportions varied so as to control the rate of evaporation of the solvent; In

the case of high atmospheric humidity a reduction in the amount of the denatured alcohol and its replacement by higher boiling solvents results inamore uniform haze-free coating.

Another composition which may be used adjvantageously consists of ethylene glycol mono ethyl ether approximately 94% by volume, tetraethylorthosilicate approximately 5% by volume,

concentrated hydrochloric acid. approximately 1% by volume. This particular composition is particularly resistant to the effects of atmose pheric humidity. It has been found also that a composition consisting ofapproximately 72% de-. natured alcohol, 7% tetraethylorthosilicate, approximately 20 ethylene glycol mono ethyl ether and approximately 1% hydrochloricacid gives excellent coatings'of high permanencean'd good efiiciency. Substantially 50% of the ethylalco ho1 maybe 'replacedby ethyl acetate if desiredi It is believed that there has been produced a micro-porous insoluble abrasion resistant layer of silicon dioxide on the surfaceof the article, the

pores being smaller in dimension than a wave of light. This micro-porous silica layer has an effective index of refraction due to its porosity ofless than 1.4 thus more nearly fulfillingthe' requirements for a layer whose index of refraction is equal to the square root of the index of refraction of the substratethan do silicon dioxide layers produced by evaporation and the like; The relative proportions under the conditions of use are such as to produce a film of this index and of such thickness and that the reflected light from one surface of the layer is approximately one half wave out of phase with the reflected wave from the other surface of the layer, or any umber of full'waves plus one half wave out of phase.

It is to be understood also that there may be incorporated in the solution substances which retain their solubilityin suitable solvents after the body of the film itself has been rendered in-, soluble. Such substances when subsequently leached out by suitable solvent treatment modify the porosity of the surface reflection reducing layer and give an additionalcontrol of the re sultant effective index of refractionof said layer. It' has been found that substances, such as urea, paraphenylenediaminedihydrochloride, glycerine, caffein hydrochloride, magnesium chloride, calcium chloride and in general other compounds soluble in the solvents used in making up the solution and'which remain uniformly and minutely dispersed throughout the result ing coating and which after the coating itself has become set and insoluble may subsequently be'removedby a solvent such as water leaving, when leached out, a coating which is considera bly more microporous than the coating which does not contain the solubleconstituents. This gives an additional reduction of the refractive index of the resulting finished coating. I While these coatings in their finishedgform are of great stability, before the heating operation or before the coatin have stood too long it is possible to remove a coating which has been damaged or which for some reason was defective, by treatment with dilute alkali in which instance the articlecanbe recoated without damage} In the solution there is a compound which is capable of being decomposedanddepositing a layer of silica, such a compound being'tetra ethylorthosilicat'e or other analagous compound and alsoin the solutionthere is'a compound ca-'- pable of decomposing 'the'decomposable silicon compounds, this decomposing agent being the acid referred to above. I In the dilute solution, as applied, it is believed that the two reacting materials do not interact rapidlyor completely but that as the solvent. isremoved the concen trations of the decomposable substance and the decomposing agent build up until concentrations are reached at which the reaction, taxesplace rapidly. At this time and -notbefore,the"co'- ascensl'i'erentzfillmor layer of'silica;isi..deposited.on the suriacel-of: thearticle: to which the. solution has. beemappliedjtheia film= subsequently being hard-- enedfeither by: aging: or by-treatment with moist heat as': described above.-

In .theexamples'given, .hydrochloriczacid is: the; decomposingzagentwhichi reacts with tetraethylorthosilioate. tor. form silicon dioxide; If these. twosingredientsz' are. mixed in concentrated form. the: mixture: becomesv hot 'andsolidifies in; a few minutes. The; solvent orsolvents. used in the disclosed solutions. serve to prevent jelling. and produce a. relatively" permanent solution usable for. aaperiod'of; time and one whichhas suitableevaporation. characteristics. The. solvent used mayrbe', of;course,.simple or complex, that is to say,- it; mayconsist. of a: single. chemical com-- pound, such; as. ethyl. acetate, alcohol. or. in fact solvent .in which the; reacting substances. can be. dissolved. The. solvent may be a complex solvent consisting of. two. or. more suitable solvents; mixed: in any; desired proportion within the rangeofworkability in which case the relati-ve evaporation rates of the solvents may differ and'the. process of building up the concentrationsofthe decomposable substance and the decomposition. agent may be-further controlled.

When surface, coatings of the typeset forth herein are-applied to. materials such as polymeriomethyl methacrylate. and the like, cellulose:acetate, cellulose; nitrate, cellulose acetatebutyrate, polystyrene-and: other resinous .or plas-- tic materials. the coating has a considerably higher; intrinsichardness than the inherent surfacehardness-of the article itself and therefore imparts an increased. resistance to scratching and abrasion. In addition, the; inert chemical nature: of the. resulting'coating and its freedom from cracks, and interpenetrating-pores serve to protecttthebody of. thearticle from attack by solvents which normally would injure it. For example: polymeric methyl methacrylate is readily attacked and dissolved by ethylene dichloride, a .commonly used solvent. When, however-,the. above. described. coating has been appliedto.-an.artic-le ofpolymeric methyl methacrylate theability of ethylene dichloride to attackthe polymeric methylmethacrylate is greatlyreduced andin some cases no attack is produced on prolonged exposure. The: coatingalso protects: the: various; plastics: against what is knownzas solvent.crazing which is a-superficial cracking ofthesurface -of; a. stressed plastic. part by; exposure-to vapors of asolvent or. to thesolvent itself AlthoughI havespecified that the: articles to betreatedare: of .atransparent natureitis to be understoodzthat any articleof opaque or transparent material might be. similarly coated; for example, an article of suitable metal might be provided with surface coatings of the above characterin which instance'the article will be renderedless visible: or: more resistant'to chemical. injury and. of. course, the same, statements apply;- to.. opaque.- glasse.S,.. lazes, paints, plastics on-irrfact-any;:substancei-crysta-lline or non-crystalliner Although.- the given" proportions are: set. forth aboye,.,it;is. to be .understoodthatv the proportions ofi the..various ingredients. might be varied in accordance withthe. procedure followed and in order. to producea. thicker. coating the content of; thesilica ester. would be increased and to produce. thinner. coatings the proportions would bedcreasediwiththeremaining. ingredients. sub.-

8;. stantially increased. or decreased proportionately. The. concentration. of; the. various. ingredients varies accordingto-the. numberof surface coatings applied. If a plurality: of surface. coatings are to be applied, itis to beunderstood that the concentrations. maybe. formed relatively weak. as.

compared with. concentrations which. are to. be applied in a single surface coating,

The coating... compositions setforth above. do

not depend upon'fortuitous decomposition of the decomposable. silicon compounds by atmospheric moisture orby moisture. or otherjsubstancesdiffusingout of the material to which the coating is applied, but the decomposition agent. ispresent inthesolution. in controlled concentration and in such aformthat until the coating has: been concentrated byevaporation after application to the. article the reactions'do nottake. place and the solution is completely stable until such time as this evaporation-takesplace. This is one of the main distinctions of the present invention overknown prior art.

The siliceous coating formedby the above. described methods may also serve as the carrier and/0r protector ofcoloring matters, such as dyes, pigments and the like. In general, spirit soluble dyes should be used in-suitable quantities to produce the desired tinctorial effect in. the finishedfilm and may be dissolved in the solution used in forming such film. Upon. evaporation of. the. solventand the completion of the hardening of the film the dyestufi or pigmentremains in. permanent form uniformly dissolved. in or scattered throughout the film. Likewise, white finely divided pigments, suchasv titanium dioxide or. other similar white. pigments having a. high index of refraction may be suspended in the film forming solution and upon the removal of the solventand completionof the formationofsuch film the particles. remain thereinand. serve as light. diffusing means.

The film containing either. the dyes or. the pigments may also. be used for modifying, the color of articles in general, mineral, metallic, or organic, whereby. a surface color of high permanence maybe applied to the article.

Incorporation of. fluorescent compounds, either insolution or asfinely divided particles in the coating solution, produce fluorescence when irradiated withradiation of suitable wave length. Inthe prior art, suchiluorescent pigments have madeuse. of organic binders, such as lacquers, varnishes, etc. which suffer from the. serious drawback of 'limited'transparency tov theradiation and lack offheatxstability and/or chemical stability. The siliceous coating having these fluorescent substances incorporated in. it is. obviously free from these drawbacks having great heat stability and high chemical permanency.

Certain inorganic compounds, such as cerium compounds, possessing thev property of absorbing ultra-violet radiation may be incorporatedinthe original solution and upon evaporation of the solution remain inthesiliceous layer;

Certain organic. compounds, not generallyconsidered as. dyestuffs, such as diphenyl, quinin, anthracene' and naphthalene, for example, may be dissolvedin the-original solution and produce upon evaporation of'the solvents films having ultra-violet absorption.

Coatings of the above character" are particularly advantageous .when used on articles having relatively smooth or highly'polished optical surfaces such, for example, as exist on lenses or other. opticalelements. In thecaseof lenses which have highly polished optical surfaces thereon such surface coatings increase the transmission of light and thereby greatly increase the efliciency of said optical elements.

It is to be understood that when applicant refers to acids throughout the specification and in the claims as a general statement, it is intended to mean any acid with the exception of hydrofluoric acid, or to mean an acid which is inert as to its effect on silica.

The composition of the solutions render it possible to apply said coatings with great ease and with great uniformity and speed without the requirement of special coating equipment and further distinguishes from most prior art coating technique in that the solutions are particularly adaptable to wide area coverage with easeand simplicity and are exceedingly transparent and particularly adaptable for coating optical elements because of the uniformity of coating which may be obtained with such composition and which will have little or no altering effect upon the desired refractive characteristics of said optical elements. In addition, the formation of such a silica film or layer is not dependent upon the chance occurrence in the atmosphere of a decomposing agent which, of course, is not controllable and produces inconsistent results. The solutions of the present invention positively control the decomposing charu Per cent Ethylene glycol mono ethyl ether approximately 24 Ethylene glycol mono butyl ether approximately 10 approximately 16 approximately 40 Butyl alcohol Denatured alcohol Tetraethylorthosilicate Concentrated hydrochloric acid approximately 2. A solution for reducing the light reflective characteristics of an article treated thereby, said solution consisting essentially of from about 1 to per cent by volume of a silicic acid alkyl ester selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof, from about 1 to 10 per cent of 2.

approximately 5 7 strong mineral acid reactable with said alkyl ester and the balance embodying substantially a mixture of an ethylene glycol lower alkyl ether, the alkyl group containing from 1 to 5 carbon atoms, and aliphatic monohydric alcohol containing from 1 to 5 carbon atoms.

3. A solution for reducing the light reflective characteristics of an article treated thereby, said solution consisting essentially of from about 1 to 10 per cent by volume of a silicic acid alkyl ester selected from the group consisting of tetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof, from about 1 to 10 per cent of a strong mineral acid reactable with said alkyl ester and the balance embodying substantially a mixture of ethylene glycol mono ethyl ether and aliphatic monohydric alcohol containing from 1 to 5 carbon atoms.

4. A solution for reducing the light reflective characteristics of an article treated thereby, said solution consisting essentially of from about 1 to 10 per cent by volume of tetraethylorthosilicate,

from about 1 to 10 per cent of a strong mineral acid reactable with said tetraethylorthosilicate and the balance embodying substantially a mixture of an ethylene glycol lower alkyl ether, the alkyl group containing from 1 to 5 carbon atoms and aliphatic monohydric alcohol containing from 1 to 5 carbon atoms.

5. A solution for reducing the light reflective characteristics of an article treated thereby, said solution consisting essentially of from about 1 to 10 per cent by volume of tetramethylorthosilicate, from about 1 to 10 per cent of a strong mineral acid reactable with said tetramethylorthosilicate and the balance embodying substantially a mixture of an ethylene glycol lower alkyl ether, the alkyl group containing from 1 to 5 carbon atoms and aliphatic monohydric alcohol containing from 1 to 5 carbon atoms.

6. A solution for reducing the light reflective characteristics of an article treated thereby consisting of:

By volume, per cent approximately '72 approximately 7 Denatured alcohol Tetraethylorthosilicate Ethylene glycol mono ethyl ether approximately 20 Hydrochloric acid approximately 1 HAROLD R. MOULTON.

REFERENCES CITED UNITED STATES PATENTS Name Date A King Dec. 7, 1948 Number 

2. A SOLUTION FOR REDUCING THE LIGHT REFLECTIVE CHARACTERISTICS OF AN ARTICLE TREATED THEREBY, SAID SOLUTION CONSISTING OF FROM ABOUT 1 TO 10 PER CENT BY VOLUME OF A SILICIC ACID ALKYL ESTER SELECTED FROM THE GROUP CONSISTING OF TETRAETHYLORTHOSILICATE, TETRAMETHYLORTHOSILICATE AND MIXTURES THEREOF, FROM ABOUT 1 TO 10 PER CENT OF A STRONG MINERAL ACID REACTABLE WITH SAID ALKYL ESTER AND THE BALANCE EMBODYING SUBSTANTIALLY A MIXTURE OF AN ETHYLENE GLYCOL LOWER ALKYL ETHER, THE ALKYL GROUP CONTAINING FROM 1 TO 5 CARBON ATOMS, AND ALIPHATIC MONOHYDRIC ALCOHOL CONTAINING FROM 1 TO 5 CARBON ATOMS. 